PROJECT ABSTRACT Constitutive activation of WNT signaling drives the growth of a broad array of human tumors, including nearly all colorectal cancers (CRCs). Despite this prominence, no Wnt inhibitors are currently approved for clinical use. The major reasons for this lack of inhibitors are the paucity of druggable WNT pathway components and the on-target gastrointestinal (GI) toxicity observed in animal models with many candidate inhibitors. Our unpublished results show that reduced expression of Casein Kinase 1? (CK1?), a negative regulator of Wnt signaling, is associated with decreased survival of CRC patients. These findings validate CK1? as a druggable target in CRC. We therefore characterized a novel, small molecule activator of CK1?, SSTC3, with pharmacokinetic properties that would allow us to target CK1? in vivo. SSTC3 attenuated CRC growth in vitro and in vivo, prolonging the survival of a mouse colorectal tumor model and inhibiting the growth of CRC xenografts. Importantly, SSTC3 did not exhibit significant GI toxicity. Thus, CK1? is a bona fide druggable target in CRC, activation of which inhibits tumorigenesis without inducing the GI toxicity that has hampered the clinical development of Wnt inhibitors. Despite this promise, many mechanistic questions remain regarding this novel class of Wnt inhibitors, which we will begin to address here. Specifically, we propose to i) determine how this class of small-molecules functions to activate CK1??activity, ii) identify the CK1? substrates that drive its efficacy but limit its effect on normal GI homeostasis, and iii) develop biomarkers that could be used to identify those CRC patients most likely to respond favorably to such inhibitors.